Control of spin-orbit torques through crystal symmetry in WTe2/ferromagnet bilayers
Control of spin-orbit torques through crystal symmetry in WTe2/ferromagnet bilayers
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DOI：
10.1038/nphys3933
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发表时间：
2017-03-01
期刊：
影响因子：
19.6
通讯作者：
Ralph, D. C.
中科院分区：
文献类型：
--
作者：
MacNeill, D.;Stiehl, G. M.;Ralph, D. C.
最近关于由重金属/铁磁体和拓扑绝缘体/铁磁体双层产生的电流诱导自旋轨道转矩的发现，为大幅度提高磁性器件的操作效率提供了潜力。然而，在迄今为止进行的实验中，自旋轨道扭矩有一个重要的限制——补偿磁阻尼的扭矩分量需要对称地位于器件平面内。这意味着自旋轨道扭矩只能驱动具有平面内各向异性的磁性器件，而不能驱动高密度应用所需的垂直磁各向异性器件的电流效率最高的磁反转（抗阻尼开关）。在这里，我们通过实验表明，这种状态不是基本的，而是可以通过使用低晶体对称性的自旋源材料来改变自旋轨道转矩在自旋源/铁磁体双层器件中允许的对称性。我们使用了过渡金属二硫化物WTe2，它的表面晶体结构只有一个镜面，没有双重旋转不变性。与这些对称性一致，当电流沿着WTe2/坡莫合金双层层的低对称性轴施加时，我们产生了面外抗阻尼扭矩，但当电流沿着高对称性轴施加时则不会。利用多层样品的晶体对称性控制自旋轨道转矩为优化未来的磁性技术提供了一种新的策略。
Recent discoveries regarding current-induced spin-orbit torques produced by heavy-metal/ferromagnet and topological-insulator/ferromagnet bilayers provide the potential for dramatically improved efficiency in the manipulation of magnetic devices. However, in experiments performed to date, spin-orbit torques have an important limitation-the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin-orbit torques can drive the most current-efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in-plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high-density applications. Here we show experimentally that this state of affairs is not fundamental, but rather one can change the allowed symmetries of spin-orbit torques in spin-source/ferromagnet bilayer devices by using a spin-source material with low crystalline symmetry. We use WTe2, a transition-metal dichalcogenide whose surface crystal structure has only one mirror plane and no two-fold rotational invariance. Consistent with these symmetries, we generate an out-of-plane antidamping torque when current is applied along a low-symmetry axis of WTe2/Permalloy bilayers, but not when current is applied along a high-symmetry axis. Controlling spin-orbit torques by crystal symmetries in multilayer samples provides a new strategy for optimizing future magnetic technologies.